Final answer:
A transaction in database systems is an atomic unit that must be fully completed or not done at all, ensuring data integrity as part of the ACID properties.
Step-by-step explanation:
The concept you're asking about, where a transaction is an atomic unit of processing, relates to the field of databases within Computers and Technology.
This characteristic is part of the ACID properties that define transaction processing systems. Atomicity means that a transaction must be treated as a single, indivisible unit of work in which all of the operations it comprises must be completed to take effect. If any operation within the transaction fails, the entire transaction fails, and the database state is left unchanged as if the transaction was never attempted.
For instance, when you perform a bank transfer, the system debits one account and credits another. Both operations - the debit and credit - comprise a single transaction. If for some reason the credit cannot be performed (perhaps due to a technical fault), the transaction will fail, and the system will roll back any partial changes, ensuring neither account is incorrectly altered.